Suzi-
I personally think SOIL TEMPERATURES have a huge effect on just when a variety starts to leaf out. For example...if you grow the same, genetically identical, variety in Florida, and in New York City, the NYC tree will lag behind the Florida tree in development. Day length may also be a factor....but the soil in Florida will warm earlier than the soil in NYC.
Containerized trees can break bud earlier when the soil temps. are consistently warmer than outside ambient temps. That's why 'the shuffle' produces results i.e ....soil is warmer during both the night and day -(garage, light bulbs, etc. heats the soil). I'll bet you can have high-intensity lights shining on a tree, 20 hours/day, with root-zone temps. near freezing, and that tree will still not break bud.
Good, documented experiments, that measure, and compare root-zone temperatures, commencement of growth, and, day lengths, are needed for containerized and in-ground trees.
Frank